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RFC 219 - User's View of the Datacomputer


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Network Working Group                                          R. Winter
Request for Comments: 219                                            CCA
NIC: 7549                                               3 September 1971
Category:
Updates: None
Obsoletes: None

                    User's View of the Datacomputer

MEMORANDUM

TO: Datacomputer Design File

FROM: R.A. Winter

SUBJECT: User's View of the Datacomputer

Date: September 3, 1971

________________________________________________________________________

Introduction

   The datacomputer is a specialized node of the ARPA network that is
   dedicated to the management of a large, shared database.  By large we
   mean several trillion bits of data, of which at least one trillion
   are on-line.  Shared may mean, for some files, shared by nearly all
   the users in the ARPA network.

   The name, datacomputer, derives from the idea that the system is
   dedicated to data handling.  Though the processor is capable of
   general computation, it will not be used for that purpose.  The
   processor, like the mass storage device, is only a component of an
   integrated system, which appears to the user as a black box.

   There is one language for addressing the black box: data language.
   This language defines everything it can do.

   All the information presented in this memorandum is about the first
   of a series of service offerings.  We use the term access method to
   refer collectively to a structure and the operations on it.  Being
   too modest to call the first one AM-1 (Access Method-1) we named it
   DCAM-1 (Datacomputer Access Method-d).  We expect subsequent DCAMs to
   generalize DCAM-1.  If the need arises, we will design parallel
   services.  All services will use the same data language.

System Overview

   The users of the datacomputer are programs running on other
   computers, retrieving data from, and storing it in, the data base.
   The environments, capabilities, and applications of these programs
   vary widely; however, a chief design goal is to allow them to share
   the data.

   There is further variation among users in physical connection.
   Remotely-located users' access is over a narrow link to the data-
   computer's low-speed port.  Local users are connected to the high-
   speed port through a link 80 times wider.

   Through its ports, the datacomputer accepts two kinds of input: data
   and requests for services.  Data is output through the ports as
   requested.

   In the data base, descriptions are stored separately from the data,
   and data elements are named, typed and ordered according to them.  A
   measure of structure independence is obtained by writing access
   requests in terms of the symbolic names of items in the data
   description.

   Directories are maintained by the system.  A hierarchical naming
   scheme is used, and access controls for privacy and data integrity
   are provided.

   Redundant copies of data and/or journals of changes are maintained by
   the system and used to effect recovery under system control in case
   of system error.  These facilities can be operated under user control
   if there is external error.

   Since the datacomputer's only interface with the outside world is
   through its ports, it sees the universe as a group of data streams.
   Specifically, these are record streams, if one views all transactions
   (in the data transfer protocol sense) as records.  Associated with
   each record stream is a data description, allowing the datacomputer
   to parse the records into named, typed elements.

   Thus all data elements--stream elements and data base--are named and
   fully described.  Data type conversion proceeds automatically, as a
   function of old and new data types, and optional information supplied
   by the user.  Reconfiguration above the element level is a matter of
   arrangement of elements in records; a full set of capabilities is
   provided for this.  In general, the using program is concerned with
   the configuration of the stream records that comprise its interface
   with the datacomputer.  The internal configuration of data affects
   the user only as it limits the data's accessibility or malleability.

   In fact, the user should not generally have to be aware of the
   internal data configuration.

   Although support on some level for all types of applications is
   attempted, the first implementation gives particular attention to
   large, simply-organized, shared files.  Emphasis is placed on
   allowing the user of such files to describe precisely what data is
   really of interest to him, so that nothing but the desired
   information is transmitted.  This is crucial for avoiding overload of
   the narrow link, and is accepted as a central design goal.

Data Base Organization

   The database contains all information stored in the datacomputer.  It
   is a set of files, which are named, physically distinct, collections
   of data.

   The location of one file, the file directory, is known to the system.
   It contains the names, locations, and certain attributes of all the
   other files.  Access to this file is restricted.

   Internally, each file has its own organization, but each organization
   is a particular application of a general model.  The particular
   application is defined by a file description associated with the
   file.

   In the general model, each file contains uniquely numbered records.
   Each record contains named fields.  A field of a certain name may
   occur more than once in a given record, and a unique number is
   associated with each occurrence.  A field contains an elementary
   piece of data, the value of the field.

   The records are variable in format and size.  Fields are variable in
   length.

   In addition to the records themselves; each file can contain an
   index.  The system maintains the index to the specifications of the
   user.  Conceptually, the index contains lists of pointers to records
   having certain properties.  A typical list might point to the records
   containing the field STATE with the value MASSACHUSETTS.

   The system supplies a unique, permanent, identifier for each record.
   This identifier maps trivially into a location in the file, or at
   worst, into a small region in which the record can be quickly
   located.  The identifier is used to pointers to the record, both from
   the index and from other records.

   Besides the physical ordering, defined by record location, a logical
   ordering will be maintained on request by the system.  This can be
   based on some simple function of record contents, such as the value
   of certain fields.  Alternatively, the user can compute the function,
   and simply supply the result (for example, by saying "insert this
   record after that one").  Retrieval from such ordered files can be
   made either in physical or logical order.

   In all such ordered files, if insertions are made, space must be
   reserved for them and garbage collection must be done periodically.
   A single field value is viewed as a homogeneous string of characters
   or basic data units.  It is described by giving the type (e.g.,
   ASCII, BIT, binary integer, etc.) and the length is some unit
   associated with the type.  When the length of a field is constant
   throughout the file, it is stored in the file description; otherwise
   it appears with each occurrence of the field.  The type of a field is
   constant.

   The information in the file description is sufficient to parse a
   record into (field name, value) pairs.  Also, given such a set of
   pairs, and a file description, the system can produce a record
   satisfying the description.  Mapping in either direction, there is
   only one possible result.

   With a record, a file description, and a (field name, value) pair to
   store in the record, there is also only one new record that can
   result.

   Thus a file description defines all the possible formats for a record
   from a particular file.

Stream Organization

   Streams are sequences of records passed from using programs to the
   datacomputer or vice versa.  The format of the records is defined as
   in the file description.  Thus streams have the same organization as
   files, except they cannot be indexed.  The operations defined on
   streams are more limited than those defined on files, since the
   records must be accessed in sequence.

   There is no concept of permanent storage for streams.  The records
   move past the datacomputer one at a time, as though they were on a
   conveyor belt.

   One record, the current record, is available to the datacomputer in
   each stream.  To begin formatting the subsequent record in an output
   stream, the datacomputer transmits the current record.  To access the
   next record in an input stream, the datacomputer relinquishes access
   to the current one.

Operations

   When the user is interested in the contents of his whole file in
   solving the problem at hand, the datacomputer's job is simple in
   terms of information retrieval.  There may be reformatting or
   reordering, but location of the right data to operate on is trivial.
   However, this will not be the standard usage of the datacomputer,
   particularly for the remote user.

   For most problems, the datacomputer expects to subset the file before
   doing anything else.  The larger the file compared to the subset, the
   less acceptable it is to transact with the full file in order to form
   the subset.  And the datacomputer will have such enormous files that
   using anything but a very small subset in one problem is most
   unusual.  Thus, subsetting without examining the entire file is a
   fundamental requirement.

   Normally, the subset will be considered formed when a list of the
   relevant record id's or record addresses is known.

   The index of the datacomputer file can be thought of as a collection
   of primitive record id lists that the file designer expected to be
   useful in forming interesting subsets.  The values of all important
   fields can be indexed.  For example, every word in a field containing
   a string of text might be indexed.  In fact, an arbitrary function of
   the contents of the record, or the relation of the record to other
   records can be indexed.

   The common logical operators (AND, OR and NOT) are defined for record
   subsets.  Arbitrarily complex expressions of them can be evaluated
   with relatively little processor time or I/O.  The ease of this
   operation results from careful design of the index and strategies,
   the most important of which is the parallel evaluation of the Boolean
   functions on large groups of records.  Certain statistical
   operations--like counting the number of records satisfying a certain
   Boolean condition--can be done directly on the index.  This can be
   used to derive question-answering strategies heuristically, or can be
   the direct input to a statistical study.

   Once the index has done all it can in subsetting, attention turns to
   the records themselves.  Certain conditions cannot be evaluated using
   the index; an obvious case is the selection of records based on the

   value of an unindexed field.  Also, certain data structures cannot be
   explicitly represented in the file:record:field model.  These must be
   constructed by the user, out of groups of records linked by pointers,
   or using other special mechanisms.  The class of operations that is
   useful in further record selection consists of field content testing,
   pointer chasing, simple computation in the numerical and symbolic
   senses, and various operations below the data element level, such as
   pattern matching, string manipulation, etc.  Such operations require
   a control structure approaching that of the general purpose higher
   level language.  It is our intention to make all of this available,
   though not with the goal of providing a computation facility, but
   rather, a data management facility that is capable of using as much
   knowledge as the programmer can supply.

   A simple set of primitives is required for file maintenance in the
   data structure we are talking about.  The operations are:

      1. add a field/record
      2. delete a field/record
      3. replace a field/record.

   The difficult part, as in retrieval, is locating the element to be
   operated on.   Notice that individual record formats can be changed
   at will: the set of possible formats is limited only by the file
   description.

   When record contents are changed, index entries that are a function
   of them must be changed also.  When the function determining what is
   to be indexed is part of the file description, the maintenance of the
   index is automatically performed by the system.  Otherwise, this is
   the responsibility of the user.

   All fields in a record can be optional, variable length, allowed to
   occur an arbitrary number of times (up to some fixed limit for each
   field).  Fields can be present and later be deleted from any record.
   Fields can be added to the file description at any time.  The only
   reason for limiting the flexibility of a record format is to reduce
   storage.

Applications

   The system outlined here is intended to be suitable for many
   applications; some examples are:

   1. Storage and retrieval of dumps and other unstructured files.  The
      system will happily pack away your one enormous record, as quickly
      and painlessly as possible.

   2. Applications that would normally be set up on tape:  sequentially
      accessed files that are copied over when they are changed.  Most
      record formats should be able to remain just as they are.  If you
      want to operate this way, the datacomputer imposes no overhead
      (such as indexing) on you.  The datacomputer willingly acts as
      unsophisticated as a tape drive; it will pass your file, adding
      and changing records as it copies them.  It will pull off the
      interesting ones, reconfigure if desired, and transmit them to
      you.  When you describe the data, you have solved the data sharing
      problem for this application.

   3. Simple-minded direct access applications.  The great hairy index
      structure neatly degenerates to imitate indexed sequential, simple
      directly-addressed files, and other old standbys in the direct
      access world.

   4. Text/document retrieval.  The indexing is made for this kind of
      applications.  In addition, documents can point to subdocuments,
      related documents, etc.

   5. Content-oriented, rapid retrieval applications are the specialty
      of the house.

   6. Large data bases used for statistical analysis or modeling such as
      the census, the common social science data bases, etc.

   7. Applications in which data element groups (such as records) are
      related in a complex fashion, and the intelligence of the
      datacomputer, which is close to the data and remote from the
      computational facility, can be put to good use.

   In all of these, an important consideration is size.  We hope to
   handle these applications properly on the datacomputer, even when the
   files are of extraordinary size.

        [ This RFC was put into machine readable form for entry   ]
        [ into the online RFC archives by Sandy Ginoza 9/2001.    ]
        [ Original has hand-written note in Postel's handwriting: ]
        [ "Received 21 Sept 71".                                  ]

 

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